The subject invention is generally directed to multiple exposure holograms, and more particularly to a technique for simultaneous exposure of a plurality of desired holograms in a single hologram recording layer with coherence mismatch between the respective sets of beams that form the different desired holograms.
In a number of applications such as laser eye protection and head-up displays, it may be desirable to utilize a hologram structure that has a bandwidth that is greater than that typically available from a single hologram. One approach to achieving a wider bandwidth hologram is to utilize a hologram structure having multiple holograms, and it would generally appear that the multiple holograms can be in the same hologram recording layer, or they can be in different layers that are individually exposed and later laminated.
However, recording multiple holograms in a single recording layer presents various problems. If the holograms are exposed simultaneously, interaction between respective sets of exposure beams (i.e., interaction of one beam from one set with one beam from another set) will produce undesirable crosstalk holograms that would reconstruct at unacceptably high intensities. Recording the multiple holograms in a single recording layer pursuant to sequential exposures can be performed with dichromated gelatin (DCG) and silver halide recording materials without significant degradation, but not with a photopolymer. A consideration with successive exposures, however, is the requirement for more handling and for DCG a change of shrinkage factor for the subsequent exposures.
Recording individual holograms on respective recording layers which are later laminated together involves the extra steps and costs associated with lamination and the additional film.
Another approach to the need for wider bandwidth is to artificially broaden the hologram in processing to obtain a wider bandwidth or even two distinct peaks. This requires critical processing for any type of recording material.